Circuits for interchanging data between time slots in a time division telecommunications system are generally known in the art, as shown in U.S. Pat. No. 3,740,483 to T. Pedersen issued June 19, 1973 for Time Division Switching System with Bilateral Time Slot Interchangers.
German Auslegeschrift No. (DT-AS) 1,278,542 and its U.S. counterpart U.S. Pat. No. 3,281,536 issued Oct. 25, 1966 disclose a network which includes one or more time switches in the form of buffer stores through which connections can be set up via a plurality of series-connected time-division-multiplex highways in time slots alternating from one time-division-multiplex highway to another. The inlet and outlet highways of the time switches have the same number of time slots.
From German Auslegeschrift No. (DT-AS) 1,295,675 as originally filed in the U.S. on Mar. 30, 1965 by P. O. Dahlman as Ser. No. 443,980, abandoned, it is known to set up connections between time-division-multiplex highways via one of a plurality of selectively usable time-division-multiplex links. Here, no time switches are provided, and the parallel connection of a plurality of time-division-multiplex links serves to avoid blocking by occupied time slots.
From No. DT-ASs 1,236,022, 1,216,377, 1,412,686, and 1,243,734 various forms of time-division-multiplex switching networks are known wherein paths can be set up via several lengths of time-division-multiplex highways to be connected together. No. DT-AS 1,216,377 (and its U.S. counterpart U.S. Pat. No. 3,029,311 issued 4/10/62 to P. W. Ward) gives a particularly detailed description of all control actions necessary to search and possibly select and occupy a free time slot.
No. DT-AS 2,103,393 discloses a circuit arrangement for switching a speech path through a time-division-multiplex exchange wherein a major number of gates are contained in an integrated-circuit module.
The object of the multi-stage switching network according to the invention is to set up connections between subscribers and junction lines to other exchanges. A special problem lies in the fact that time-division-multiplex exchanges require some central control circuitry, so it is important to allow for good possibilities of expansion which involves no disproportionate expense incurred by the expansion of the central control.
The switching network according to the invention is characterized in that all time switches are of uniform construction and perform a conversion from a time-division-multiplex highway with a small number of channels to a time-division-multiplex highway with a large number of channels and conversely, that a plurality of time switches are connected in groups and in parallel to at least one of the time-division-multiplex highways with a large number of channels, and that paths between two time-division-multiplex highways with an equal number of channels extend through two series-opposed time switches between which the path extends via a time-division-multiplex highway with a different number of channels. The uniformity of the time switches permits a higher production rate and, thus, a saving in cost. In addition, the amount of circuitry required in the central control of the exchange is reduced, because all expansions and additions necessitate only the addition of identical partial functions. Compared to this, the cost and complexity of two series-opposed time switches at places where a simple slot change would be sufficient is not so high.
A special development is characterized in that the time switches contain memories for PCM-coded information and cyclic control memories which permit a set-up path to be cyclically switched through the time switches without the aid of a central control. This integration of the control memories is only possible because, once a path has been set up, all time switches operate in the same way irrespective of their specific switching functions.
As a rule, the time-division-multiplex highways with a small number of channels will transfer bits serially, and the time-division-multiplex highways with a large number of channels will transfer bits in parallel over several conductors.
This limits the additional conductors required to short lengths of highway, while serial transfer takes place on all possibly longer lengths of highway.
A special development of the switching network according to the invention is characterized in that each group of time switches connected in parallel at the output end has access to a plurality of selectively usable time-division-multiplex highways with a large number of channels. This increases reliability and the traffic volume capable of being handled within a group.
A special development of the invention is characterized in that internal subscribers of the exchange are connected to the time-division-multiplex highway with a small number of channels via sampling gates, or that said time-division-multiplex highway leads as a junction to another exchange or to a concentrator. This freedom of wiring results from the uniform operation of the time switches. The number of subscribers connectable to a time-division-multiplex highway with a small number of channels is limited by the number of channels.
There are two possibilities of establishing a new connection. One possibility is to control the establishment of a new connection from a central control which selects and occupies usable time-division-mulitplex highways and time slots on the basis of destination information fed in over the subscriber lines or in a data channel of the junction.
Another possibility is to establish a new connection in two steps, in the first of which the control of the time switch concerned searches for, selects and occupies a free path to a register without any intervention by a central control, and in the second of which a free path to said destination is searched for, selected and occupied on the basis of the destination information evaluated in the register. The switching network according to the invention is particularly flexible when expansions are necessary; on the one hand, it is characterized by the fact that all changes of traffic intensity are effected by suitably interconnecting time-division-multiplex highways of a small number of channels via time switches connected to time-division-multiplex highways of a large number of channels and by subsequent splitting into a greater or smaller number of time-division-multiplex highways of a small number of channels via time switches connected in series opposition. On the other hand, it is characterized by the fact that all gradings and group-selection stages between time-division-multiplex highways of a large number of channels are formed by a corresponding split up into time-division-multiplex highways of a small number of channels via time switches and by a subsequent combination into destination-group-forming time-division-multiplex highways of a large number of channels via time switches connected in series opposition.
A special development of the switching network according to the invention is characterized in that the time switches, including the associated control circuits, are made using integrated circuit technology. In this instance, the uniform design of all time switches is particularly cost-saving.